The aim of this study was to determine the significance of isolation of Aspergillus fumigatus from cultures of respiratory specimens in nongranulocytopenic patients. The medical records of patients with respiratory specimens positive for Aspergillus fumigatus who were admitted to an adult pneumology ward were reviewed during a 2-year period. A total of 80 respiratory specimens from 76 patients yielded Aspergillus fumigatus. Forty-eight patients were colonized with Aspergillus fumigatus, whereas the 28 (37%) remaining patients had pulmonary aspergillosis, manifest as aspergilloma ( n=19 patients), chronic necrotizing pulmonary aspergillosis ( n=7 patients), and bronchial aspergillosis ( n=2 patients). The presence of typical hyphae in direct examination of bronchoscopic specimens was more likely to be found in infected than in colonized patients ( P=0.04). No immunological test was positive in colonized patients, whereas anti- Aspergillus antibodies were detected in 55% of infected patients ( P<0.001). Pulmonary tuberculosis was the most common underlying lung disease in patients with aspergilloma, but it was not found in any patient with chronic necrotizing pulmonary aspergillosis ( P=0.006). Anti- Aspergillus antibodies were more likely to be detected in patients with aspergilloma (78%) than in patients with chronic necrotizing pulmonary aspergillosis (14%) ( P=0.007). The analysis of predisposing factors, in conjunction with immunological tests and examination of bronchoscopic specimens, is helpful in distinguishing between colonization and infection with Aspergillus fumigatus, as well as for differentiating between aspergilloma and chronic necrotizing pulmonary aspergillosis.